1. Field of the Invention
The present invention relates to a power supply system and an image forming apparatus, by means of a piexoelectric transformer-type of a power supply unit.
2. Description of the Related Art
Conventionally, in an image forming apparatus of an electrophotography system, a DC voltage is applied to a transfer member to electrostatically transfer a toner image from an image bearing member to a recording medium. For an excellent transfer, a current of about 10 μA is usually supplied to the transfer member at a high voltage (voltage greater than several hundred V which is higher than a commercial power supply voltage). Conventionally, a power supply unit uses a winding electromagnetic transformer to generate such a high voltage. However, the electromagnetic transformer is an obstacle for miniaturization and weight reduction of the power supply unit. Therefore, a piezoelectric transformer (piezoelectric ceramic transformer) is used in place of the winding electromagnetic transformer.
Therefore, the use of the piezoelectric transformer has an advantage of allowing miniaturization and weight reduction of the power supply unit. The power supply unit using the piezoelectric transformer includes a piezoelectric transformer that outputs a maximum voltage at a predetermined resonant frequency and a generation unit that generates a signal that oscillates at a drive frequency for driving the piezoelectric transformer across a predetermined frequency range including the resonant frequency.
Such a piezoelectric transformer-type power supply unit is disclosed in Japanese Patent Application Laid-Open No. H11-206113. An example of applying the apparatus to an image forming apparatus is disclosed in Japanese Patent Application Laid-Open No. 2007-068384.
However, in the case where the piezoelectric transformer-type power supply unit is used in the image forming apparatus, there are the following problems.
When the frequency of the piezoelectric transformer is controlled, secondary vibrations of high frequency components are generated if the drive frequency reaches a high frequency area, and an unstable area is generated in the output characteristics. The output voltage is unstable if the piezoelectric transformer is controlled at a drive frequency of the unstable area. More specifically, the output of a low voltage with a high drive frequency is unstable in the piezoelectric transformer-type power supply unit. Therefore, a lower limit voltage that can ensure the stable output voltage (hereinafter, referred to as “ensured lower limit voltage”) is set in consideration of variations in circuit configurations of individual power supply units. In other words, the frequency that drives the piezoelectric transformer is a drive frequency that falls within a predetermined drive frequency area that does not include the frequency of the unstable area. In the power supply unit that drives the piezoelectric transformer at the frequency of the predetermined drive frequency area, a voltage between the maximum voltage and the ensured lower limit voltage is set as an capable output voltage.
Therefore, in image forming apparatuses by means of the conventional type of piezoelectric transformer-type of a power supply unit, there is a problem in which the output voltage for outputting voltage to the power-fed member in a stable condition is limited to the voltage whose magnitude ranges between the maximum voltage and the ensured lower limit voltage